1. Field of the Invention
This invention relates to axial flow machine rotating blades and specifically to root tangs used to reduce rocking of the blades.
2. Description of Related Art
A typical turbine rotor assembly of a gas turbine engine has a plurality of rotor or more particularly turbine blades extending radially outward across a working fluid flowpath. Typical turbine blades generally include an airfoil section mounted on a platform which in turn is held radially outward of a blade root section by a blade shank. The blade is normally mounted in the rim of a rotor disk by its root interlockingly engaging a slot cut in the rim. The blade may have a mid-span shroud as is typical of fan blades or a tip shroud such as may be found in low pressure turbine blades.
One typical type of rotor blade has an axially extending root that is curvilinear in form and referred to as a dovetail root. The root is designed to be axially slid into a matching conforming slot referred to as a dovetail slot. Formed between the slots are posts in the rim of the disk which are adapted to engage and hold tangentially extending lobes or tangs of the dovetail root by the centrifugal force developed during engine operation.
Single tang dovetail and multiple tang fir tree roots are used throughout the turbomachinery industry as a means of attaching blades to disks. Multiple tang fir tree roots have radially stacked sets of tangentially and axially extending tangs and, due to their basic design and tolerance, minimize the amount of blades axial and tangential rocking in its slot due to the upper and lower tangs acting as a moment restraint. Single tang dovetail roots, however, limit axial and tangential rotation only when the dovetail curvilinear cross-section form comes in contact with its corresponding disk dovetail slot surface, or adjacent blade platforms or shroud edges come in contact, or the blade shank contacts the disk, all of which result in larger tolerance stacks therefore increasing the amount of blade rotation.
Minimizing the amount of blade tangential rocking is important to minimize dovetail wear. Particularly important to blade designs having mid-span or tip shrouds is the need to minimize the amount of the disengaging of adjacent blade shrouds and shroud disengagement is a function of the amount of dovetail axial and tangential rocking motion.
The problem is particularly acute at low rotational turbine rotor speeds where the centrifugal forces holding the tangs against their corresponding inwardly facing dovetail slot surfaces is marginal at best and the blades can rock or angle over in the tangential direction.